 .box {
 	width: 400px;
 	position: relative;
 	height: 398px;
 	border: 1px solid #ccc;
 	margin: 100px;
 }
 
 .mask {
 	display: none;
 	position: absolute;
 	top: 0;
 	left: 0;
 	width: 300px;
 	height: 300px;
 	background-color: yellow;
 	opacity: .5;
 	border: 1px solid #ccc;
 	cursor: move;
 }
 
 .big {
 	display: none;
 	position: absolute;
 	left: 410px;
 	top: 0;
 	width: 500px;
 	height: 500px;
 	background-color: red;
 	/* 提升显示层级 */
 	z-index: 999;
 	border: 1px solid #ccc;
 	overflow: hidden;
 }
 /* 必须给图片加定位才能移动 */
 
 .big img {
 	position: absolute;
 	top: 0;
 	left: 0;
 }